The intention was to allow the user agent to use modal UI for display
selection. In that scenario, it would be complicated to support
simultaneous requests, or confusing to service one request immediately
after the other.
I'll craft a PR along the lines of,
> If there is already an unsettled Promise from a call to `start()` on
any PresentationRequest in the same controlling browsing context,
return a new `Promise` rejected with an `OperationError` exception and
abort all remaining steps.
Alternatively, we could convert this step to an implementation note
that browser is allowed to use modal UI for display selection and
allowed to reject a simultaneous `start()` request according to the
constraints of that UI.)
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